This invention relates to signal processing, and more particularly, to signal processing techniques for use in Digital Audio Broadcasting (DAB) systems.
Digital Audio Broadcasting is a medium for providing digital-quality audio, superior to existing analog broadcasting formats. Both AM and FM DAB signals can be transmitted in a hybrid format where the digitally modulated signal coexists with the currently broadcast analog AM or FM signal, or in an all-digital format without an analog signal. In-band-on-channel (IBOC) DAB systems require no new spectral allocations because each DAB signal is simultaneously transmitted within the same spectral mask of an existing AM or FM channel allocation. IBOC promotes economy of spectrum while enabling broadcasters to supply digital quality audio to their present base of listeners. Several IBOC DAB approaches have been suggested. One such approach, set forth in U.S. Pat. No. 5,588,022, presents a method for simultaneously broadcasting analog and digital signals in a standard AM broadcasting channel. Using this approach, an amplitude-modulated radio frequency signal having a first frequency spectrum is broadcast. The amplitude-modulated radio frequency signal includes a first carrier modulated by an analog program signal. Simultaneously, a plurality of digitally-modulated carrier signals are broadcast within a bandwidth which encompasses the first frequency spectrum. Each digitally-modulated carrier signal is modulated by a portion of a digital program signal. A first group of the digitally-modulated carrier signals lies within the first frequency spectrum and is modulated in quadrature with the first carrier signal. Second and third groups of the digitally-modulated carrier signals lie outside of the first frequency spectrum and are modulated both in-phase and in-quadrature with the first carrier signal. Multiple carriers are employed by means of orthogonal frequency division multiplexing (OFDM) to bear the communicated information.
FM IBOC broadcasting systems using have been the subject of several United States patents including U.S. Pat. Nos. 5,465,396; 5,315,583; 5,278,844 and 5,278,826. In addition, a commonly assigned pending patent application for a “Method and System for Simultaneously Broadcasting and Receiving Digital and Analog Signals, by D. Kumar and B. Hunsinger, Ser. No. 08/274,140, filed Jul. 1994 discloses an FM IBOC DAB system, now U.S. Pat. No. 5,956,624.
The signals used in Digital Audio Broadcasting are subject to fading and noise (interference). Digital Audio Broadcasting receivers may include Viterbi decoders. Conventional implementations of soft-decision Viterbi decoders rely on constant signal and gaussian noise statistics for (near) optimum decoding. Practically these statistics should be nearly constant over the path memory of the Viterbi decoder, or the span of the interleaver, whichever is greater. An interleaver may be used to yield statistical independence of the fading statistics of the soft symbols over the path memory of the Viterbi decoder after deinterleaving. The remedy for a flat fading channel causing fluctuating signal levels with constant noise is well known. However, there exists a need for a signal processing technique that can address independently varying signal and noise levels.